Did they keep the 20mm's or the mg's together with the 6-pounder gun ?
They kept the 303s. The Mollins 57mm cannon filled the bomb bay leaving no room for the 20mm cannons (which are installed in the front half of the VI's bomb bay).
The Mosquito XVIII "tze-tze" also had extra armor and was thus heavier than our VI model (and tougher!). On the other hand, the Mollins is a proper AP anti-tank auto-cannon that saw action as a land weapon. The XVIII was initially intended to be a tank killer. In the end, though it was built in small numbers, the XVIII saw extensive use with coastal command against ships and U-boats. It was later phased out when the Mossies VI's were equipped with 25lbs, solid heads AP rockets (not the ones in the game) which were even more effective against ships, and still allowed to carry the 4*20mm. The Mollins was also fired in air-to-air on a few occasions and at least one JU88 was downed.
The XVIII will be a fine and unique addition to AH and the best anti-tank flying cannon in the game. There are more urgent planes to add (Beaufighter!), but if you rate them by "fun" or "coolness" or "added value" to the game, this one will be at the top...